**1. Introduction**

With the continuous increase in the world population, which may reach over 9 billion by the year 2050, global food production needs to increase by an estimated 50% at least to keep pace with this population increase and meet its nutritional needs [1]. Fish and fishery products can meet a significant proportion of the world's food needs by 2050 [2]. In this context, it was noted that the world per capita fish consumption increased from an average

**Citation:** Sheikha, A.F.E.; Allam, A.Y.; Oz, E.; Khan, M.R.; Proestos, C.; Oz, F. Edible Xanthan/Propolis Coating and Its Effect on Physicochemical, Microbial, and Sensory Quality Indices in Mackerel Tuna (*Euthynnus affinis*) Fillets during Chilled Storage. *Gels* **2022**, *8*, 405. https://doi.org/ 10.3390/gels8070405

Academic Editor: Miguel A. Cerqueira

Received: 3 May 2022 Accepted: 21 June 2022 Published: 25 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of 9.9 kg in the 1960s to 20.5 kg in 2018 [3]. Although fish has high commercial value, it is an extremely perishable product. Mackerel tuna (*Euthynnus affinis*) is a commercially important fish in high demand. As with other fish, Mackerel tuna is easily and rapidly damaged (highly perishable), so it is very susceptible to quality degradation. The major factor causing quality degradation in fish, including mackerel tuna, is microbial activity, even though the first changes are caused by the endogenous enzymes of fish, which ultimately shortens their shelf life [4–6]. Microbial deterioration proceeds fast because of the presence of large amounts of low-molecular-weight compounds, high water activity, and high post-mortem (pH > 6) in fish muscles. Hence, cooling is necessary to prolong the shelf life of fish and is often combined with vacuum packaging to prevent the growth of aerobic microflora that cause spoilage [5,7]. Thus, the application of appropriate packaging and/or processing techniques will be the best solution to extend the shelf life of fish and fish products [8]. In addition to the short shelf life, another challenge facing fresh fish consumption is that seafood products take a long time to prepare as meals [9]. Despite freezing and cold storage being significant and frequent methods of preserving fish and fish products, they cannot completely prevent chemical and oxidation reactions in fish and fish products [10]. The reason for the occurrence of these reactions may be related to the presence of polyunsaturated fatty acids in fish and fish products, which oxidize rapidly in the presence of oxygen [11]. Therefore, the use of preservatives, especially natural ones, has become an urgent need to extend the shelf life of perishable foods such as fish [12].

Propolis (bee glue) is a balsamic product obtained from exotic Africanized bees *Apis mellifera* L. [13]. Propolis extract is well-known for its functional properties, such as anti-inflammatory, pharmacological, antiviral, anticancer, antioxidant, antifungal, and antibacterial activities [14]. Natural preservatives benefits have recently been enhanced by incorporating them into various edible coatings and films on food products [15,16]. Due to the edible coatings' simplicity and eco-friendly nature. Several characteristics distinguish the edible coating. It acts as a carrier for bioactive components and is a semi-permeable barrier to moisture loss, gas exchange, and oxidative reactions [17,18]. Several studies have been conducted showing the possibility of using natural gums such as xanthan in formulating edible coatings and improving their characteristics [19–22].

Xanthan gum is a polysaccharide produced by *Xanthomonas campestris*, and a food additive that is commonly added to foods as a thickener or stabilizer [23,24]. Xanthan is featured because of its ability to enhance food flavor, consistency, texture, shelf life, and appearance [24]. Xanthan has multiple technological advantages that make it a rich raw material with various applications, especially for food. The features of xanthan could be listed as follows [23,25]: (1) high viscosity at low concentrations: for example, a solution with a concentration of 1% appears almost gel-like at rest, yet pours readily and has a very low resistance to mixing and pumping; (2) high resistance to a wide pH range (2–12) makes xanthan well-suited to foods; (3) high thermal stability; the viscosity is not affected by temperatures in the range of (0–100 ◦C), and it has excellent freeze-thaw ability; (4) high solubility of xanthan gum renders it appropriate for many applications, including foods; (5) high compatibility with most of the commercially available thickeners.

Furthermore, one of the new food applications of xanthan is its use as a gelling agent in edible coating building [26].

Bioactive edible coatings or films from natural preservatives with antioxidant and antibacterial properties, prolong the shelf life of fish and fish products [27]. The main advantage is that the edible film helps in the reduction of environmental pollution [28,29]. In this sense, there is just one study that indicates the effect of using edible coatings on extending the shelf life of mackerel tuna fish (*Euthynnus affinis*) fillets [30]. Kumar and others' [30] study aimed to develop a bioactive edible coating from gelatin and chitosan, incorporated with different concentrations of clove oil as a natural preservative, and evaluate their effect on the shelf life of mackerel fillets under refrigerated conditions (4 ◦C). This edible coating demonstrated its potential as a natural antibacterial agent which can be used for packaging tuna and other fishery products.

Due to the features of propolis as a natural preservative (antioxidant and antimicrobial agent), it could be integrated with xanthan gum in formulating gel-based edible coating [31] to extend the shelf life of fish and fishery products. There is no published data on the application of a xanthan/propolis composite coating to preserve fish and fish products' quality and shelf life. Therefore, to our knowledge, this is the first paper to study the effects of using a composite edible coating from xanthan containing various levels (0, 1, and 2%) of ethanolic extract of propolis (EEP) on the physicochemical, microbiological, and sensory quality parameters of mackerel tuna fish (*Euthynnus affinis*) fillets during chilled storage (2 ◦C) for 20 days.
